<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
    </ul>
    <script>
        var ul = document.querySelector('ul')
        //
        var lili = ul.children[0].cloneNode(true);   //把ul里的第一个li克隆放到lili
        ul.appendChild(lili)
    </script>
 
</body>

</html>